In recent years, recording apparatuses using disks and the like have increased in memory capacity and data transfer speed. This, in turn, has required a disk rotating device for use in such a recording apparatus to be capable of high-speed and high-precision rotation. To this end, a hydrodynamic bearing having a central shaft supported at its opposite ends as disclosed in U.S. Pat. No. 5,504,637 is used in a rotary main shaft of the recording apparatus.
A conventional hydrodynamic bearing will hereinafter be described with reference to FIGS. 7 and 8.
FIG. 7 illustrates a recording device employing the hydrodynamic bearing.
A sleeve 30 provided in the center of a hub 29 is rotatably fitted around a stationary shaft 22 having one end fixed to a lower casing 21. Disks 35A, 35B, 35C, 35D as recording media are attached to the hub 29 as being spaced from each other by spacers 36A, 36B, 36C.
A flange member 24 is attached to the other end of the stationary shaft 22 by an upper shaft 28 as being fitted in a step portion 30A of the sleeve 30. The upper shaft 28 has a male thread, which is threaded in the other end of the stationary shaft 22 so that the flange member 24 is press-fitted to the other end of the stationary shaft 22.
A thrust plate 27 which is opposed to an upper face of the flange member 24 and an outer circumference of the upper shaft 28 is fixed in a recessed portion 29A of the hub 29.
One set or, typically, two sets of herringbone grooves 23A, 23B are provided in at least one of an outer circumferential portion of the stationary shaft 22 and an inner circumferential portion of the sleeve 30. An inner spiral groove 26 is provided in either one of a surface of the step portion 30A of the sleeve 30 and a face of the flange member 24 which are opposed to each other, and an outer spiral groove 25 is provided in at least one of opposed faces of the flange member 24 and the thrust plate 27. These grooves 23A, 23B, 25, 26 and an oil pit 30B are filled with a lubricant 31.
A motor rotor 33 is fixed to the hub 29, and a motor stator 32 is fixed to the lower casing 21. Further, an upper casing 34 is attached to the upper shaft 28.
In the conventional hydrodynamic bearing, the motor rotor 33 starts co-rotating with the hub 29, the sleeve 30, the thrust plate 27, the disks 35A, 35B, 35C, 35D and the spacers 36A, 36B, 36C, when the motor stator 32 is energized to develop a rotating magnetic field.
At this time, the herringbone grooves 23A, 23B collect the lubricant 31 to generate a pressure by pumping action, and the outer spiral groove 25 and the inner spiral groove 26 also collect the lubricant 31. The pressure thus generated causes the hub 29 to rotate in a completely non-contacting state with respect to the stationary shaft 22.
However, the aforesaid arrangement has the following drawbacks.
Since the inner diameter D2 of the thrust plate 27 is slightly greater than the inner diameter D1 of a bearing portion of the sleeve 30 as shown in FIG. 8, the lubricant 31 filled in the herringbone grooves 23A, 23B flows out through the step portion 30A and scatters over the thrust plate 27 as indicated by 31A due to a centrifugal force exerted thereon during high speed rotation.
Further, when air which is dissolved into the lubricant through air-liquid interfaces 38A, 38B during the rotation is accumulated in the oil pit 30B and the like and grows into bubbles 37A, 37B, 37C, 37D, the air-liquid interfaces 38A, 38B are bulged, so that the lubricant 31 flows out from the upper side as indicated by 31A and 31B, and from the lower side as indicated by 31C in FIG. 8. This results in depletion of the lubricant 31 in the herringbone grooves 23A, 23B, the outer spiral groove 25 and the inner spiral groove 26.